Allow self-referencing closures.

frog/tests/leg_only/src/Closure4Test.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+closure0() {
+  var f = fib(i) {
+    if (i < 2) return i;
+    return fib(i - 1) + fib(i - 2);
+  };
+  Expect.equals(5, f(5));
+}
+
+closure1() {
+  var decr1 = 0;
+  var decr2 = 0;
+  var f = fib(i) {
+    if (i < 2) return i;
+    return fib(i - decr1) + fib(i - decr2);
+  };
+  decr1++;
+  decr2 += 2;
+  Expect.equals(5, f(5));
+}
+
+closure2() {
+  var f = fun(doReturnClosure) {
+    if (doReturnClosure) {
+      return inner() => fun(false);
+    } else {
+      return 499;
+    }
+  };
+  var g = f(true);
+  Expect.equals(499, g());
+}
+
+main() {
+  closure0();
+  closure1();
+  closure2();
+}